Structural Refinement of Disordered Zeolites using the PDF Method

1. Structural Refinement of Disordered Zeolites using the PDF Method María Martínez, Raúl F. Lobo Department of Chemical Engineering University of Delaware Inmaculada Peral Center for Neutron Research NIST Thomas Proffen Los Alamos Neutron Science Center LANL ACNS June 6, 2004

2. Zeolites • Crystalline aluminosilicates with a 4-connected tetrahedral framework structure enclosing cavities occupied by large ions and water molecules, both of which have considerable freedom of movement, permitting ion exchange and reversible dehydration (Smith, 1988) • Most important class of solid acids • Important Environmental Applications • Catalytic Converters (Diesel and Gasoline) • DeNOx Catalyst in Power Plants • VOC Removal • Structure-property relation require detailed atomic information

3. Zeolite Mordenite

4. Why so much interest in Zeolites? • Well-defined molecular structure • Establish structure-property relationships • Prediction of properties from structure • BUT… • Disorder is an inescapable fact on zeolitic materials • Structure-Property relations are not always simple

5. Disorder in Zeolites • Framework Substitution • Other atoms besides Si • Extra-Framework Cations • Partial occupancies, multiple cations • Adsorbed Species • No translational symmetry • Lower symmetry than framework • Framework Disorder • Static • Dynamic • Opportunity for the PDF method

6. Previous Application of PDF Method to Zeolites • Cs atoms in Siliceous Zeolites • Petkov, V. et al., PRL, 2002 • Coordination of CHCl3to Acid Sites • Eckert, J. et al, JACS, 2002

7. Static Disorder • Two-Dimensional Translational Symmetry • Disordered stacking of ordered layers • ABC-family of Zeolites • One-Dimensional Translational Symmetry • Disordered arrangement of ordered ‘tubes’ • ZSM-48, SSZ-31 and others • Zero-Dimensions • Description must be Statistical

8. Building ZSM-48 Using 1D Units

9. ZSM-48: Experiment and Simulation a=0.2 b=0.1 g=0.8 Experiment Anis. Broadening Isotropic Broad

10. Objectives • Determine the suitability of the PDF method to investigate the local structure in selected zeolite systems • Use zeolite beta as a test case • Zeolite beta: • 9 Si and 16 O atoms in asymmetric unit

11. Zeolite Beta • Important alkylation catalyst • 12-ring (~7.5 Å) 3-D Pore system • Intergrowth of two polytypes Polytype A Polytype B

12. Polytype B

13. XRD of Beta Simulations with DiFFAX

14. Views of the Periodic Building Unit (PerBU) 6.6 Å View down c axis View down a or b axis Periodic Building Unit of Beta • Basic building element is the Periodic Building UnitPerBU • Both polytes A and B can be described from the same PerBU

15. Highly resolved spectra are obtained 9-T sites are distinguished PerBU of Polytypes A and B should be the same 29Si NMR Spectra of Zeolite Beta Siliceous Beta (Reported) Camblor et al. Chem.Comm. 1996

16. Sample Preparation • From B-Beta • B removed in acid media • T-vacancies are not healed • From Zn-Beta • Highly hydrophobic sample • No defects (by NMR) • Preferred sample

17. 29Si NMR Spectra of Zeolite Beta Siliceous Beta (Reported) Our Sample Camblor et al. Chem.Comm. 1996

18. Scattering Measurements • Neutron PDF • GLAD, IPNS (good statistics 20 Å-1) • NPD, LANL (good statistics up to 30 Å -1) • X-ray PDF • APS, l=0.1573 Å • Data treatment with ATLAS, PDFGetX and PDFGetN • Refinement with PDFFit

19. Simulated PDF of polytypes of Zeolite Beta (as reported)

20. After energy minimization (GULP) the PDF of both polytypes is nearly identical Comparison of experimental measurement and minimized structure match well NPDF Beta: Energy Minimization

21. NPDF vs. XPDF • NPDF was not sufficient • XPDF is better but gives distorted tetrahedra • Need to use scattering length ratios at Q=4.4 Å-1fSi/fO=2.2 gives good fit of first two peaks

22. Progress of RefinementPolytype A • Comparison of starting and final models after refinement • Initial Rw=33.7% • Final Rw=20.3%

23. Refinement of Polytype B • Comparison of starting and final models after refinement • Initial Rw=30% (XPDF) • Final Rw=20.0 % • Final Rw=20.8% (NPDF)

24. Refinement of the NPDF

25. Refined bond distances

26. Conclusions • The PDF method can be used for the refinement of the PerBUs of zeolite structures provided: • Sample is compositionally simple • Both Neutron and X-ray data sets are refined sequentially (or simultaneously) • Complexity of the PerBU is about 9-T atoms or less

27. Application of PDF to Dynamic Disorder • Zeolite Chabazite • Large negativethermal expansioncoefficient • Diffraction suggestsSi-O-Si linkages arecontracting • Others suggest rockingof rigid tetraherda

28. Chabazite: RT Experiment and Model

29. PDF of Chabazite • Data consistent with rocking of rigid tetrahedra in sample • Mechanim of contraction still unknown

30. On going studies… • Use Rigid-Unid-Modes (RUMs) to analyze the motion of tetrahedra in the zeolite • Based on the RUMs, predict most stable configurations and determine if these are consistent with PDF data

31. Acknowledgments • APS, IPS, LANL • Sarvjit Shastri • Chris Benmore • Valeri Petkov • Thomas Proffen • Sven Vogel • NSF for funding